No, it’s not unexpected. There is always a surplus in April when tax bills are due. For many many decades. We will have another large annual deficit at the end of September, which is what matters.
I completely understand your point, but the article says that the market (whatever that means in this context) expected a deficit. So, if there is a surplus, it was unexpected by these fools.
It would be nice if there was a small annual deficit in 9/17, but I’m not holding my breath.